use split-gpg for dev env

[sssoleileraaa]

Description

Update docs once how we want to move forward with freedomofpress/securedrop-client#497

Background

The developer environment on Qubes does not use split-gpg. Instead it uses a development gpg private key inside a gpg keychain that is stored in the temporary configuration directories. This is also how it works in a non-Qubes environment.

It might make sense to use split-gpg with the key configured in the sd-gpg AppVM instead. This could make it easier to maintain (perhaps not if we still have to maintain the dev key and setup code for non-Qubes development) and also to understand the differences between running python -m securedrop_client with proxy vs ./run.sh without proxy. The only difference should be whether or not we use the securedrop-proxy RPC service and Tor is used.

[gonzalo-bulnes]

Covered by existing docs on setting up development environments.